Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

It is well known that babies born extremely prematurely run a higher risk of a host of developmental problems, with the chief causes thought to include infections contracted in the womb and bleeding in the brain.

The new research shows the strong degree to which infections contracted after birth — including infections not directly involving the brain — are linked to neurological problems later, said lead author Dr. Barbara Stoll of Emory University.

About 47 percent of children in the study who had had an infection had at least one developmental problem by age 2, compared with 29 percent of children who had not had an infection.

“This study shows us that successfully treating an extremely low birth weight infant’s infection does not automatically ensure that the infant will do well,” said Dr. Duane Alexander, director of the National Institute for Child Health and Human Development, which funded the study. He said premature infants who have had infections should be watched carefully for developmental problems.

Tiny premature infants are especially vulnerable to infections because their immune systems have not matured and because ventilators and other medical machinery create a pathway for germs to enter the body.

The study, published in Wednesday’s Journal of the American Medical Association, involved 6,093 babies born weighing less than 2.2 pounds.

Sixty-five percent of them had at least one infection while hospitalized, and 41 percent had at least one mental or physical impairment at follow-up visits between the ages of 1 and 2.

The study also found:

A 40 percent higher risk of cerebral palsy in children who had had sepsis, or infection in the bloodstream.

A 2.4 percent higher risk of motor impairments, including delayed walking, in those who had had sepsis plus an intestinal infection called necrotizing enterocolitis.

A 2.2 percent higher risk of vision impairment in those who had had meningitis, or inflammation of the brain.

Cerebral palsy involves permanent damage to parts of the brain that control movement. Most cases are thought to result from events before birth, including infections in the mother. Other causes include brain infections. Lack of oxygen during birth, once thought to be a major cause, only accounts for a small number of cases.

The findings provide “a critical opportunity to examine whether neurodevelopmental impairment can be reduced by reducing rates of infection,” Dr. Michael Msall of the University of Chicago said in a JAMA editorial.

Strict hand-washing by doctors and nurses and increased efforts to wean tiny babies from feeding tubes and other machinery may help, Stoll said.